The invention relates to a method for operating a two-stroke engine as disclosed, for example, in published international patent application WO 00/55488.
The two-stroke engine disclosed in the above-identified publication includes two fuel paths. One of the fuel paths opens into the crankcase, whereas the other fuel path feeds into a storage channel. The distribution is so provided that, at idle, fuel is supplied in large part by the crankcase; whereas, at full load, essentially the entire fuel flows in via the storage channel. This allocation of fuel leads to an advantageous balanced operating performance with excellent exhaust-gas values but it has been determined that there is a poor run-up of the engine during accelerations.
It is an object of the invention to provide a method of the type described above which is so improved that a powerful acceleration of the engine is ensured.
The method of the invention is for operating a two-stroke engine including a drive motor in a portable, handheld work apparatus including a chain saw, a brushcutter and a cutoff machine. The engine includes: a cylinder having a cylinder wall; a piston mounted in the cylinder to undergo a reciprocating movement along a stroke path between top dead center and bottom dead center during operation of the engine; the cylinder and the piston conjointly delimiting a combustion chamber; a crankcase connected to the cylinder; a crankshaft rotatably mounted in the crankcase; a connecting rod connecting the piston to the crankshaft to permit the piston to drive the crankshaft as the piston reciprocates in the cylinder; an exhaust outlet for conducting exhaust gases away from the combustion chamber; a storage channel having a first end in the form of an inlet into the combustion chamber and the inlet lying approximately opposite to the exhaust outlet; the storage channel having a second end communicating via a controlled opening with the crankcase; a first fuel metering device connected to the storage channel between the first and second ends thereof; the first fuel metering device and the storage channel conjointly defining a first fuel path; the crankcase having a crankcase inlet; a second fuel metering device connected to the crankcase inlet and defining a second fuel path therewith; a transfer channel connected between the crankcase and the combustion chamber; the method comprising the steps of: providing an acceleration unit operatively connected to the first and second fuel paths; and, causing the acceleration unit to pump additional fuel into each of the first and second fuel paths during an acceleration of the engine.
It has been determined that the otherwise undertaken distribution of the fuel supply in the case of an acceleration is to be overcontrolled at least for a time in order to convey an additional quantity of fuel into each of the two fuel paths. This ensures a disturbance-free run-up of the engine in the case of acceleration.
It is practical during acceleration to pump approximately 30% to 60%, especially approximately 40%, of the additional fuel quantity to be supplied into the crankcase path. A greater part of the fuel is thereby supplied via the transfer channel and ensures an excellent charge of the combustion chamber.
In an advantageous further embodiment of the invention, the storage channel path is connected to the crankcase path via the idle chamber of a carburetor mounted in the crankcase path. The carburetor is especially a membrane carburetor. The acceleration unit pumps into the idle chamber in order to raise the fuel quantity in the storage channel.
The acceleration unit can be driven by the pressure difference between the crankcase and the control chamber of the carburetor. A configuration of the acceleration unit as a pump, which is controlled by the crankcase pressure, is advantageous.